1. Field of the Invention
This invention pertains to the treating of wells and, more particularly, to a method for selectively restricting the flow of fluids through perforations in a deviated oil well casing by small balls or spheres of appropriate size.
2. Description of the Prior Art
It is common practice in drilling oil and gas wells to deviate the wellbore from the vertical. When the wellbore is intentionally deviated from the vertical, it is called directional drilling. Directional drilling has application in several situations such as producing from inaccessible locations (i.e. populated areas, hostile environments, under rivers, etc.), drilling from offshore platforms, and sidetracking a vertical wellbore after the original well was drilled into water-bearing formations or after downhole problems require abandonment of the lower portion of the wellbore.
It is common practice in completing oil and gas wells, including deviated wells, to set a string of pipe, known as casing, in the well and to pump cement around the outside of the casing to isolate the various formations penetrated by the well. To establish fluid communication between the hydrocarbon-bearing formations and the interior of the casing, the casing and cement sheath are perforated.
At various times during the life of the well, it may be desirable to increase the production rate of hydrocarbons by acid treatment or hydraulic fracturing. If only a short, single, hydrocarbon-bearing zone in the well has been perforated, the treating fluid will flow into this productive zone. As the length of the perforated zone or the number of perforated zones increases, treatment of the entire productive zone or zones becomes more difficult. For instance, the strata having the highest permeability will most likely consume the major portion of a given stimulation treatment leaving the least permeable strata virtually untreated. Therefore, techniques have been developed to divert the treating fluid from the high permeability or undamaged zones to the low permeability or damaged zones.
Various mechanical techniques for selectively treating multiple zones have been suggested including techniques using, for example, packers, baffles and balls, bridge plugs, and ball sealers.
Packers have been used extensively for separating zones for treatment. Although these devices are effective, they are expensive to use because of the associated workover equipment required for the tubing-packer manipulations. Moreover, mechanical reliability tends to decrease as the depth or deviation of the well increases.
In using baffles and balls to separate zones, a baffle ring, which has a slightly smaller inside diameter than the casing, fits between two joints of casing so that a large ball, or bomb, dropped in the casing will seat in the baffle. After the ball is seated in the baffle, the ball prevents further fluid flow down the hole. One disadvantage of this method is that the baffles must be run with the casing string. Moreover, if two or more baffles are used, the inside diameter of the bottom baffle may be so small that a standard perforating gun cannot be used to perforate below the bottom baffle.
A bridge plug, which is comprised principally of slips, a plug mandrel, and a rubber sealing element, has been run and set in casing to isolate a lower zone while treating an upper section. After fracturing or acidizing the well, the plug is generally retrieved, drilled, or knocked to the well bottom with a chisel baller or drillpipe. One difficulty with the bridge plug method is that the plug sometimes does not withstand high differential pressures. Another problem with this technique is that the placement and removal of the plug can be expensive due to rig costs and associated equipment.
One of the more popular and widely used diverting techniques uses ball sealers. In a typical method, ball sealers are pumped into the well along with the formation treating fluid. The balls are carried down the wellbore and to the perforations by the fluid flow through the perforations. The balls seat upon the perforations and are held there by the pressure differential across the perforations.
Although ball sealer diverting techniques have set with considerable usage, the balls often do not perform effectively because only a fraction of the balls injected actually seat on perforations. Ball sealers having a density greater than the treating fluid will often yield a low and unpredictable seating efficiency, highly dependent on the difference in density between the ball sealers and the fluid, the fluid viscosity, the flow rate of the fluid through the perforations, and the number, spacing and orientation of the perforations. The net result is that the plugging of the desired number of perforations at the proper time during the treatment to effect the desired diversion is left virtually to chance.
Lightweight ball sealers are ball sealers having a density less than the treating fluid density and have been successfully used to improve seating efficiency. The treating fluid containing lightweight ball sealers is injected down the well at a rate such that the downward velocity of the fluid is sufficient to impart a downward drag force on the ball sealers greater in magnitude than the upward buoyancy force of the ball sealers. Once the ball sealers have reached the perforations, all will seat and plug the perforations provided fewer balls are injected than there are perforations accepting fluid, thereby forcing the treating fluid to be diverted to the remaining open perforations. Although these lightweight ball sealers can be highly effective in improving diversion, one problem with using these ball sealers occurs when the downward flow of fluid in the casing is so slow that the drag forces exerted on the balls by the treating fluid may not overcome the upward buoyancy force of the ball sealers and thus the ball sealers may not be transported to the perforations. This problem is generally experienced during treatments pumped at low rates and in particular matrix treatments such as matrix acidizing.
One prior method of selective diversion is disclosed in U.S. Pat. No. 4,194,561. This method involves the use of placement devices for positioning buoyant ball sealers at a specific location within the wellbore. These devices are equipped with means to prevent the upward migration of the buoynat ball sealers past the placement device. The ball sealers are seated on the perforations by flowing fluid down the casing and through the device. These devices are normally used to selectively close the perforations located at the lowermost region of the casing.
Another prior art method for selective diversion is disclosed in U.S. Pat. No. 4,287,952. This method involves the selective sealing of perforations at the top or bottom of the deviated casing (wherein "top" and "bottom" are identified with reference to an imaginary plane which is aligned substantially vertically and extends along the longitudinal axis of the casing). Other perforations are formed away from these top and bottom perforations thereby permitting balls of particular densities to seat on such top or bottom perforations, leaving the other perforations placed away from these top and bottom perforations open for fluid communication with a zone to be treated.
Yet another prior art method of placing ball sealers onto a casing perforation is disclosed in U.S. Pat. No. 4,195,690. This method involves the placement of a plurality of balls at a transition region between a first and second fluid. This method has application in a deviated wellbore also as a means of negating the effects of gravity/buoyancy forces which limit the seating capabilities of buoyant and nonbuoyant ball sealers to those perforations preferentially located on the high and low side of the pipe, respectively. However, U.S. Pat. No. 4,195,690 does not teach the number of ball sealers to be used in sealing at least one perforation in a deviated wellbore if the balls are transported down at an interface between two immiscible fluids.
Therefore, there still exists a need for an improved method of treating a specific zone in a deviated casing without the need to be concerned about the circumferential location of the perforations around the casing or about the use of a placement apparatus but, rather, based on more proven diversion techniques using lightweight ball sealers.